Process for regenerating a bath for chemical etching of titanium parts

ABSTRACT

Disclosed is a method of regenerating a nitric and hydrofluoric acid bath contained in a machining vessel, the method including, when the etching bath is spent, performing steps of: transferring a portion of the spent etching bath, referred to as the “spent” solution, from the machining vessel into a reactor; adding NaF and NaNO3 to the spent solution, to form HF, HNO3, and Na2TiF6; separating the resulting precipitate from the supernatant; transferring the supernatant, which is a regenerated solution, into a tank; measuring the concentrations of HF, of HNO3, and of dissolved titanium in the tank and in the machining vessel; and determining the volume of regenerated solution that can be added to the spent etching bath to obtain a regenerated bath in which the concentrations of HF, of HNO3, and of dissolved titanium lie in acceptable concentration ranges, and transferring the regenerated solution into the machining vessel.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the field of methods of fabricating metal parts by chemical machining, and more particularly to the field of treatment baths for chemically etching metals and metal alloys.

The invention provides a method of regenerating a nitric and hydrofluoric acid bath for chemically etching parts made of titanium or titanium alloy, for the purposes of eliminating the titanium dissolved in the bath and of recycling the HF and HNO₃ acid reagents with reduced environmental impact.

Description of the Related Art

Titanium, whether on its own or alloyed, is used in the fields of space and aviation because of its excellent mechanical characteristics associated with remarkable resistance to corrosion and good strength at high temperatures. The chemical machining technique is commonly used in the fabrication of parts, whether they be simple or complicated, which technique consists in removing material by etching with acid to dissolve titanium chemically. This may be done over the entire part in order to adjust its thickness. This may also be performed selectively, over certain portions of a part, by applying a localized mask. It is thus possible to lighten a structure, e.g. a panel, by removing a few tens of micrometers to several millimeters of material. This need to lighten parts is of major importance in the sector of manufacturing aircraft (airplanes, helicopters).

Chemical etching is also used to prepare the surfaces of titanium parts, but over thicknesses that are smaller (0.1 millimeters (mm) at most, and in general about 30 micrometers (μm)), as applies when pickling and decontaminating parts. In the description below, the term “machining” is used to cover both preparing surfaces and also machining proper, since these techniques are very similar.

The technique of chemical machining consists in immersing a part in an etching bath for a length of time that is appropriate and controlled, in order to obtain the desired removal of material. Use is generally made of a solution of hydrofluoric acid (HF) and of nitric acid (HNO₃), which appears to be the only medium that is capable of dissolving this material, which is particularly inert chemically. As reported in the literature, titanium and titanium alloys may be subjected to various reactions in a nitric and hydrofluoric acid bath. Some of them involve ionic forms of titanium, which become complexed with fluoride ions. Finally, it is accepted that the two most-likely reactions can be written as follows:

3Ti+4HNO₃+12HF→3TiF₄+4NO+8H₂O  (1)

Ti+4HNO₃+6HF→H₂TiF₆+4NO₂+4H₂O  (2)

In these processes, hydrofluoric acid acts to etch the metal by corrosion, while nitric acid acts to passivate and to catalyze the reactions. The concentrations of these two acids vary as a function of the desired objective, with a bath having more HNO₃ being used for pickling, while a bath with a higher concentration of HF is used for chemical machining.

The parts involved in these treatments are titanium that is pure, i.e. having a degree of purity greater than 99%, or else an alloy having a majority of titanium. By way of example, the various types of titanium that can be dissolved in such a bath are pure titanium such as T40 (99.4% purity), or titanium alloys such as TA6V (6% aluminum and 4% vanadium) and Ti6242 (6% aluminum, 2% tin, 4% zirconium, and 2% molybdenum).

The chemical machining process removes material from the part, thereby raising the concentration of dissolved titanium in the bath while consuming the HF and HNO₃ acids. Various parameters influence the use and the effectiveness of the bath in production conditions.

Firstly, it is known that baths for chemically machining titanium operate over well-defined concentration ranges of the HF and HNO₃ acids. It is commonly accepted that baths are functional for an HF concentration in the range 0.5N to 1N and an HNO₃ concentration in the range 1.4N to 1.8N. Care is thus taken to add these reagents regularly in order to keep them at effective contents.

Furthermore, while parts are being treated, the bath picks up titanium, thereby reducing the efficiency of the titanium-dissolving reaction. Specifically, because of the solubility limits of the chemical species of titanium, the bath loses its effectiveness above a certain concentration of titanium. It is also observed that the surfaces of parts deteriorate, thereby leading to a risk of intergranular corrosion of the material, associated with hydrogen embrittlement. As a result, above a certain concentration of titanium dissolved in the bath, the parts that are produced are not in compliance for the intended use because of defects that appear on all of the parts. Operators deciding on acceptable efficiency levels generally set the maximum at 30 grams per liter (g/L) or 40 g/L of titanium. The chemical machining bath is then said to be “spent”.

In order to return to satisfactory production conditions, the bath needs to be replaced by a new solution after the bath has been drained off, thereby requiring production to be interrupted for several hours. That does indeed eliminate the excess titanium, but without it being recovered, and with a considerable loss of HF and of HNO₃, which are chemicals that are very expensive. Spent baths need to be stored on site, and then disposed of in compliance with the regulations in force relating to the treatment of toxic waste, which requires logistics that are burdensome, expensive, and not environmentally friendly. As an additional difficulty, very few vehicles are suitable for transporting this corrosive liquid waste.

In order to avoid stopping the equipment completely, which penalizes production, it is possible to drain off part of the bath, so as to eliminate a fraction of the solution, and thus return to acceptable concentrations of dissolved titanium. On each occasion, new quantities of HF and of HNO₃ reagent needed to be added in order to adjust their concentrations in the bath. By this method, it is possible to reduce the duration of interruptions to production, but such interruptions happen more frequently, and above all the spent solutions still need to be eliminated with the same major economic and environmental consequences.

In order to remedy those drawbacks, proposals have been made to treat baths so as to recover the components therefrom and make them usable for some other use. Nevertheless, such methods have been abandoned, both for economic reasons and also because of the dangerousness of the acids concerned.

Proposals have also been made to regenerate spent baths so that they can be used in a new cycle of treating titanium parts. One method of regeneration relies on adding a potassium salt so as to form a titanium compound that is highly insoluble in water, thereby forming a precipitate that can be eliminated by settling and/or filtering the solution. That operation may be carried out on a fraction of the solution of the bath, which is drawn off and treated in dedicated equipment.

In particular, it is known to obtain potassium hexafluorotitanate (K₂TiF₆), by adding potassium nitrate (KNO₃). As described in Document FR 2 946 364, this reacts with the dissolved titanium to form nitric acid (HNO₃) and simultaneously the solvated salt K₂TiF₆(H₂O), which tends to precipitate in an aqueous medium. In similar manner, it is known to add potassium fluoride (KF), which leads to the formation of hydrofluoric acid (HF) and of K₂TiF₆(H₂O). The precipitated salt of potassium hexafluorotitanate is then separated from the liquid phase, which is returned to the machining vessel. This reduces the concentration of dissolved titanium in the bath, while also resupplying it with reagents.

Choosing to use potassium salts as reagents relies on the fact that the resulting salt, K₂TiF₆, is practically insoluble in water, unlike other salts such as sodium or ammonium hexafluorotitanate, and still worse calcium or aluminum hexafluorotitanate, which are considerably more soluble. The K₂TiF₆ salt precipitates in a few hours at ambient temperature, after which it can easily be separated by filtering. The filtrate containing the regenerated acids together with a quantity of titanium that has not reacted, can then be returned to the etching vessel.

Nevertheless, that method presents a major drawback, which can make it unsuitable for aviation applications. Specifically, the presence of potassium compounds is not explicitly authorized in the treatment of materials, nor in methods of fabricating parts that are for use in aircraft manufacture. However, in spite of all the precautions that are taken, it is very difficult to completely avoid having any potassium in the regenerated solution, in particular because the reaction between the dissolved titanium and the potassium salts KF and KNO₃ is not total. Some potassium thus remains in the medium, persisting in small quantities in soluble form in the filtrate, and ending up in the chemical machining vessel. Since the potassium ions that are introduced are once again in the presence of titanium in the bath, they tend to precipitate in the form of K₂TiF₆. That method is therefore not used by the manufacturers of parts for the aviation industry, nor indeed in other sectors for which quality and safety criteria are high.

There therefore exists a need for a method that is suitable for use in the aviation industry and that serves to regenerate the nitric and hydrofluoric acid baths that are used for chemically etching titanium parts. An object of the present invention is to satisfy this need by proposing a method that enables the titanium that is dissolved in the bath to be eliminated, and that enables the HF and HNO₃ acid reagents to be regenerated. Another object of the present invention is to enable a method that satisfies these requirements to be performed on a solution extracted from the bath, but without requiring a prolonged interruption of production. Another object of the invention is to enable regenerated acids to be returned to the etching bath without running the risk of introducing therein elements that are undesirable, such as potassium. In general manner, an object of the present invention is to provide a method of regenerating baths for machining titanium that is safe and reliable in terms of the quality of the parts produced, that complies with environmental regulations, that is easy to perform, and that is inexpensive.

SUMMARY OF THE INVENTION

The present invention satisfies these objects by a method in which a spent solution is caused to react with sodium salts, specifically sodium nitrate (NaNO₃) and sodium fluoride (NaF), in order to form sodium hexafluorotitanate (Na₂TiF₆) together with the etching acids HNO₃ and HF, and in compliance with the following reactions:

2NaNO₃+H₂TiF₆→Na₂TiF₆+2HNO₃  (3)

2NaF+H₂TiF₆→Na₂TiF₆+2HF  (4)

Since sodium hexafluorotitanate (Na₂TiF₆) dissolves to some extent in water, it does not precipitate in full, so it is difficult to eliminate it from the aqueous phase. Specifically, sodium hexafluorotitanate salts present solubility that is considerably higher than that of potassium salts, specifically 65 g/L for Na₂TiF₆ compared with 12 g/L for K₂TiF₆, at ambient temperature (H. Ginsberg, Z. Anorg. Allgem. Chem. 204, 225 (1932) cited by V. J. Landis; J. H. Kaye; Battelle Pacific Northwest Labs Richland Wash., in Radiochemistry Of Titanium, Ft. Belvoir Defense Technical Information Center, January 1971). A significant proportion of Na₂TiF₆ thus remains dissolved in the solution for returning to the machining vessel.

Now, in the past, the presence of this complex of sodium in the bath has been considered as being harmful for proper conduct of the machining, because of the risk of precipitation, with particles becoming deposited on the parts that are being treated in the bath.

In the context of the present invention, it has been found, on the contrary, that it is possible to take advantage of the particular properties of this compound: part of the salt Na₂TiF₆ precipitates, thereby enabling a large portion of the titanium to be eliminated by settling, while the fraction that remains in solution can be found in the machining bath, where it runs the risk of accumulating over time and precipitating. However, in unexpected manner, in the tests that have been performed, the risk of forming solid deposits that would pollute the metal surfaces of parts during their chemical treatment did not materialize. On the contrary, the salts of sodium and of titanium remained soluble in the machining bath up to high contents, without disturbing the process and without precipitating.

Another major advantage of the method of the invention is that the presence of sodium ions in the machining vessel has also been found not to constitute an obstacle to the proper conduct of the method. Specifically, since the reaction of titanium with NaNO₃ and NaF is not complete (reactions (3) and (4) above), these species remain in solution at non-negligible contents, most likely in dissociated form (sodium ions Na⁺ together with NO₃ ⁻ and F⁻). However, it has been found that the presence of sodium in the etching bath does not disturb the chemical etching process and that the parts produced comply with the required specifications. Furthermore, sodium is one of the compounds for which harmlessness has been established for aviation applications. There is therefore no need to seek to eliminate it at all costs from the solution when regenerating spent baths.

The regeneration process is thus not subjected to the constraint of totally eliminating sodium, whether in free or complexed form, and it is possible to envisage complete recycling of the machining bath. A manufacturer can thus act on various other parameters in order to optimize the regeneration method.

Thus, the present invention provides a method of regenerating a nitric and hydrofluoric acid bath for chemically etching parts made of titanium or titanium alloy in a machining vessel, the method comprising determining whether said etching bath is spent, and if so, performing the steps consisting in:

a) transferring a portion of the spent etching bath, referred to as the “spent” solution, from the machining vessel into a reactor;

b) adding a quantity of NaF and a quantity of NaNO₃ to the spent solution, and allowing it to react to form HF, HNO₃, and Na₂TiF₆;

c) settling to separate the resulting precipitate from the supernatant;

d) transferring the supernatant, which is a regenerated solution, into a tank;

e) measuring the concentrations of HF, of HNO₃, and of dissolved titanium in the tank and in the machining vessel; and

f) determining the volume of regenerated solution that can be added to the spent etching bath in order to obtain a regenerated bath in which the concentrations of HF, of HNO₃, and of dissolved titanium lie in respective predefined acceptable concentration ranges, and transferring said volume of regenerated solution into the machining vessel.

The presently-defined method may be performed as often as necessary for regenerating the spent etching bath over long periods of treating various parts made of titanium or titanium alloy. A complete cycle, i.e. the succession of steps a) to f), may be repeated sequentially, however it may also be conducted in overlapping manner. Under such circumstances, and by way of example, steps e) and f) of one cycle may be performed at the same time as steps b) and c) of a following cycle.

In each cycle, a portion of the spent bath, referred to as the “spent solution”, is transferred into a reactor. The regeneration reagents, namely NaF and NaNO₃, are added to the spent solution, where they react with the titanium that is present therein. The reaction produces HF, HNO₃, and Na₂TiF₆ (step b). In step c), a precipitate, essentially containing solvated Na₂TiF₆, is formed and settles. It can be eliminated as such from the bottom of the vessel (of the reactor or of the settling tank). The supernatant is the regenerated solution, which contains essentially HF and HNO₃, a certain amount of titanium and of Na⁺ that has not reacted, and finally a proportion of Na₂TiF₆, as explained in detail below. In step e), the various concentrations involved are measured in order, in step f), to calculate the volume of regenerated solution that can be added to the spent etching bath in order to obtain a regenerated bath suitable for performing effective chemical etching of parts made of titanium or of titanium alloy.

Such a method enables spent solutions to be regenerated in order to recycle them, while recovering the acid reagents and while returning them to the bath for treating titanium parts. As a result, there is a significant reduction in the amounts of the acids that need to be added for reconstituting a functional bath.

According to a characteristic of the method of the present invention, in step b), NaF and NaNO₃ are added in quantities that are proportional to the molar quantities that correspond to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium. The quantities of NaF and of NaNO₃ that need to be introduced are determined in proportion to the quantity of titanium present in the bath, and in compliance with the reactions of above-described equations (3) and (4). It should be observed that the two salts NaNO₃ and NaF may be added together or separately. Furthermore, their relative proportions may be modulated so that one of the reactions (3) or (4) predominates, depending on the quantities of each of the acids that it is desired to regenerate.

NaF and NaNO₃ may be added in proportional quantities that are equals to the molar quantities for stoichiometric reactions of NaF and NaNO₃ with the dissolved titanium. For stoichiometric molar quantities, it is necessary to add two moles of NaF for one mole of dissolved titanium, and likewise two moles of NaNO₃ for one mole of dissolved titanium.

NaF and NaNO₃ may also be added in proportional quantities that are less than the stoichiometric quantities. For example, in step b), NaF and NaNO₃ may be added in quantities that are 2% to 8% less, preferably about 5% less, in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium. As a result, it is ensured that there is no excess sodium in the reaction medium, since even though its presence need not be excluded from the machining bath, it is preferable to avoid it accumulating and to conserve conditions that are close to known conditions. Furthermore, even though an industrial method might be subjected to drift in measuring and monitoring appliances, that is of little consequence so long as a safety margin is provided.

In accordance with the invention, in step b), the mixture is preferably allowed to react under stirring for a period of 2 hours (h) to 4 h at a temperature lying in the range 25° C. to 40° C. In more preferred manner, the reagents NaF and NaNO₃ are introduced into the reactor while the solution is at a temperature similar to the machining temperature (in the range 35° C. to 45° C.), thereby enabling the amount of regeneration to be optimized by dissolving the reagents. The reaction continues for 2 h to 4 h under stirring in order to obtain the best efficiency for transformation into Na₂TiF₆. In an aqueous medium, a portion of this compound precipitates, and on settling it forms a sludge.

In a preferred implementation of the method of the present invention, in step c), the settling is performed at a temperature that is positive but less than or equal to 15° C. Also preferably, during settling, the solution is cooled to a temperature lying in the range 5° C. to 15° C. in order to optimize the efficiency of precipitation and in order to limit the quantity of Na₂TiF₆ that remains in solution and that could be transported to the machining vessel where it would accumulate, or that could clog filters downstream from the reactor, as described below. Thereafter, the sludge is drawn off, e.g. via a stopper arranged in the bottom of the reactor (or of the settling tank).

The reaction step b) and the settling step c) are performed in succession, and they may take place in the same reactor, with its temperature being adjusted accordingly. Alternatively, the content may be transferred from the reactor into a settling tank. The person skilled in the art has no difficulty in selecting one implementation or the other, e.g. as a function of the available production conditions and equipment. The method of the invention may thus be performed in such a manner that settling takes place in said reactor, or after the content of the reactor has been transferred into a settling vessel.

According to a preferred characteristic of the invention, the resulting regenerated solution may be filtered at the inlet or at the outlet of said tank, using a device suitable for retaining chemical species of size greater than 5 μm. Specifically, in step b) and c), precipitation and settling eliminate most of the Ti in the form of Na₂TiF₆(H₂O), i.e. they eliminate about 90% to 98% by weight, depending on the conditions of the reaction medium. Up to 10% of Na₂TiF₆ may thus remain dissolved in the solution, which it is preferable to eliminate by providing for the regenerated solution to be filtered, even though tests had been carried out that show that the Na₂TiF₆ remains essentially in soluble form. A filter having a mesh of 5 μm suffices for retaining titanium complexes. Such filters are commercially available.

It is specified at this point that it is also possible to provide for filtering the chemical machining bath by setting up circulation through the machining vessel. Specifically, in the particularly unfavorable event of a large quantity of sodium being introduced into the chemical machining bath, it is possible to filter the bath continuously in order to eliminate the Na₂TiF₆ salts that form. It should nevertheless be observed, that this is a situation that is extreme and improbable given its solubility.

Once the solution has been regenerated, it is stored in said tank with other volumes of regenerated solution and is subsequently returned to the machining vessel (step f). In advantageous manner, the regenerated solution is heated to a temperature identical to the temperature of the etching bath prior to being poured into the machining vessel. This provision enables the etching bath to be brought quickly up to temperature in order to reduce the length of time during which machining is stopped while reconstituting the chemical etching bath.

Insofar as a portion of the spent bath is drawn off from the vessel in order to be subjected to regeneration, e.g. 10% of the spent bath, an equivalent volume needs to be added in step f) in order to reconstitute the bath. It is possible to act on this parameter by adding a quantity of regenerated solution, and then making up the quantity with water. It is advantageous for the regenerated solution that is added to the machining vessel to serve to reconstitute an etching bath that is effective. For this purpose, it is desirable for the contents of HF and of HNO₃ to lie within acceptable predefined ranges. According to a preferred characteristic of the invention, the acceptable range of concentrations for HF in the machining bath extends from 0.5N to 1N (i.e. from 10 g/L to 20 g/L); and

-   -   the acceptable range of concentrations for HNO₃ in the machining         bath extends from 1.4N to 1.8N (i.e. from 88 g/L to 113 g/L).

In practical manner, the regenerated solution needs to be analyzed in order to determine the concentrations of the species present and in order to predict their impact on the composition of the bath that is about to be reconstituted. Also, after step f) of transferring the volume of regenerated solution into the machining vessel, it is possible to monitor the concentrations of HF and of HNO₃ in said machining vessel and to adjust them as a function of the respective concentrations desired in the etching bath.

Ideally, these concentrations should be as high as possible within their respective ranges. If the concentrations are low within their ranges, HF and HNO₃ should be added to the bath in order to optimize its composition for effective chemical etching. It should be observed that since chemical etching of titanium consumes much more HF (6 HF for 1 titanium), more HF is regenerated than HNO₃, and thus more HF is returned to the etching bath.

In particularly advantageous manner, the present invention enables numerous regeneration cycles to be carried out without loss of time. It is thus advantageous to work with an etching bath in which the titanium content oscillates between values that are close together and selected to lie in an optimized range having the best efficiency. Specifically, it is known that the presence of dissolved titanium has an influence on the efficiency of the machining reactions. Even though regeneration is it possible to eliminate most of the titanium in the precipitated sludge, it is recommended to ensure that the titanium remaining in the regenerated solution that is to be added to the solution remaining in the machining bath does not exceed acceptable limits. That is why, in accordance with another preferred characteristic of the invention, the range of titanium concentrations that are acceptable in the machining bath extends from 10 g/L to 40 g/L. Naturally, its concentration should preferably be minimized, in order to avoid rapidly reaching the limit value for triggering a new regeneration operation.

In even more preferred manner, the range of concentrations that are acceptable for titanium in the machining bath extends from 18 g/L to 25 g/L. This means that a regeneration cycle is triggered more frequently, but that the bath is maintained in an operating range that is more effective, i.e. in which the efficiency of the chemical etching reaction is optimized relative to the content of dissolved titanium. Until now, this implementation could not be envisaged, since draining off a still-operational etching bath at 25 g/L of titanium would constitute a significant waste of reagents. It can be seen that by regenerating the etching solutions and by using them to reconstitute baths instead of discarding them as waste, the method of the invention makes it possible to optimize chemical machining both from a quality point of view and from an economic point of view.

Throughout the above description, it is assumed that the spent baths being regenerated are specifically nitric and hydrofluoric acid baths. The term “spent bath” is used to mean an etching bath in which the dissolved titanium content departs from a range of values that are predefined as being acceptable. The range of acceptable values may be defined by the person skilled in the art as a function of the availability of machining and regeneration equipment, as a function of its capacity, as a function of efficiency targets, etc., and it preferably lies within the above-described values. Thus, in accordance with the invention, it is determined whether the etching bath is spent by the operations consisting in:

-   -   measuring the concentration of dissolved titanium in the etching         bath;     -   comparing the measured concentration with said predefined range         of acceptable concentrations; and     -   if the measured concentration is greater than the maximum value         of said range, triggering step a) of transferring a portion of         the spent etching bath from the machining vessel to said         reactor.

As explained above, the range of acceptable titanium concentrations may extend from 10 g/L to 40 g/L, or indeed from 18 g/L to 25 g/L.

The spent solution that is drawn off from the machining vessel is advantageously stored in a buffer vessel, possibly together with spent solutions that have previously been drawn off, prior to being transferred into the reactor, so that step a) is performed in two stages. The following regeneration steps b) to f) are not started until a sufficient volume has been collected. It is thus possible to draw off solution from the spent bath on several occasions before starting the regeneration operation.

Although the tests that have been carried out show that the presence of dissolved sodium in the machining bath does not have a negative effect on the quality of the machining, it is recommended to monitor the sodium content in the machining bath so as to ensure that it does not accumulate in exaggerated manner. This avoids any risk of undesirable secondary reactions taking place. In particular, it is desirable to avoid a possible reaction between sodium and the other reagents such as titanium and fluorine, which might give rise to reprecipitation and impede proper operation of the process (by depositing particles on surfaces, clogging ducts, . . . ). That is why, in accordance with a preferred characteristic of the invention, the concentration of sodium in the etching bath is measured, and if said sodium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel. Preferably, said predetermined limit value for the concentration of sodium in the machining bath is no greater than 7 g/L, and is preferably less than 5 g/L.

This operation may be carried out systematically prior to transferring a portion of the spent etching bath constituting a spent solution into the reactor, or else at some other rate, as determined by the person skilled in the art. Also, although under certain particular reaction conditions (that have not yet been encountered) the presence of sodium might lead to the formation of an Na₂TiF₆ sodium salt in the bath, it is possible to remove it without major difficulty. To do this, and as mentioned above, the machining vessel may be provided with a filter integrated in a recirculation system for the purpose of eliminating solid particles and the largest molecules.

When the parts being machined are made of titanium alloy, vanadium is very often present. It accumulates little by little in the bath, so it is preferable to limit any such progression. Use is made of the same principle as that described above, with a fraction of the spent bath being eliminated when the concentration of vanadium becomes too high. That is why, the method of the invention may include an operation in which the concentration of vanadium dissolved in the etching bath is measured, and if said vanadium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel. The person skilled in the art can set this limit value, e.g. at a level of the order of a few grams per liter to about ten grams per liter.

Thus, it has been found that nitric and hydrofluoric acid baths for chemically etching titanium and its alloys can be regenerated using the sodium compounds NaF and NaNO₃. It is thus possible, in a single operation, to reduce the concentration of dissolved titanium in the bath by eliminating the precipitate of Na₂TiF₆, and to return towards a satisfactory concentration of the acids in the bath. In unexpected manner, and even though a fraction of the sodium does not react with the dissolved titanium and even though all of the dissolved titanium salt Na₂TiF₆ is not eliminated from the solution by settling, it is possible to reconstitute machining baths that are effective and to manufacture parts made of titanium and its alloys that comply with the requirements of the aviation industry. The nitric and hydrofluoric acid baths in question are both baths for chemical etching, strictly speaking, and also baths for pickling parts made of titanium or titanium alloy.

Using sodium salts reduces the constraints on the regeneration method. This makes it possible to act on other parameters in order to optimize the method from technical, environmental, and economic points of view.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be better understood, an associated details appear in the light of the following description given with reference to the accompanying figures, in which:

FIG. 1 is a diagram of equipment of the invention for regenerating chemical etching baths; and

FIG. 2 shows crystals of the Na₂TiF₆ precipitate as obtained after washing and drying the regeneration sludge, and as seen using a scanning electron microscope.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1: Regeneration Techniques

A machining vessel one hundred containing an etching bath based on HNO₃ and on HF receives parts made of titanium or titanium alloy. As machining operations progress, the bath picks up titanium, while HNO₃ and HF are consumed. These concentrations are measured at a regular time intervals in order to monitor the state of the bath and to restore its state, should that be necessary. For each compound, acceptable concentrations correspond to a range of concentrations that are predetermined as being those concentrations within which it is desired to conduct the machining of parts. The acceptable ranges given below are given by way of example for a bath for machining parts made of titanium or titanium alloys.

-   -   Ti: 10 g/L to 40 g/L (better 18 g/L to 25 g/L);     -   HF: 10 g/L to 20 g/L; and     -   HNO₃: 88 g/L to 113 g/L.

Since the etching reagents HNO₃ and HF are consumed continuously, an adjustment is made each time that is necessary, whenever at least one of them is at the bottom limit of its acceptable range, with this being in spite of—and independently of—any addition constituted by returning regenerated solution.

The bath is considered as being spent, either when its titanium concentration reaches 40 g/L (or 25 g/L if opting for higher reaction efficiencies and shorter operating cycles), or else when its concentration is going to reach this limit during the next operation of machining parts. A portion of the spent bath, e.g. amounting to 10% thereof, is then transferred to the regeneration circuit.

In practice, it is advantageous to estimate the rate at which the concentration of titanium increases in the bath, in order to anticipate the moment when it is going to reach the upper limit of the acceptable range. It is also possible to evaluate the frequency at which it is necessary to transfer spent solution to the regeneration circuit, and to proceed with transferring a portion of the bath regularly at said frequency, thereby facilitating automation. When the method is automated, a buffer tank 110 may be interposed between the chemical etching vessel 100 and the regeneration reactor 1, so as to pour the spent solution therein at the appropriate time, and trigger a regeneration operation subsequently, without any need to synchronize those two actions.

The spent solution is poured into the reactor 1, either directly from the machining vessel 100 or else from the buffer tank 110. The regeneration reagents NaF and NaNO₃ are then introduced into the reactor 1 via feed pipes 30 and 31 provided for this purpose. The quantities added are calculated as a function of the titanium content in the spent solution, on the basis of the molar quantities of reactions (3) and (4). They are determined stoichiometrically for a total reaction minus 1% to 8% so as to avoid the machining bath containing too much NaNO₃ and NaF that has not reacted with titanium in the reactor 1. These compounds could react with the titanium in the machining vessel 100 and precipitate, which it is desired to avoid, even though it appears that precipitating a small quantity of titanium salt does not lead to problems with chemical machining. Table 1 below gives the quantities of reagents that are to be added as a function of the quantities of titanium dissolved in the spent solution (minus 5% relative to stoichiometric reactions).

TABLE 1 Concentrations of NaNO₃ and of NaF added as a function of the content of titanium in the spent solution [Ti] in (g/L) [NaNO₃] (g/L) [NaF] (g/L) 18.0 60.8 30.1 20.0 67.6 33.4 22.0 74.4 36.7 24.0 81.1 40.1 26.0 87.9 43.4 28.0 94.6 46.8 30.0 101.4 50.1 35.0 118.3 58.45 40.0 135.2 66.8

The reactor one is maintained at a temperature selected in the range 25° C. to 40° C. for a period of 2 h to 4 h, while stirring. In order to optimize the reaction, it is preferable to select a temperature close to 40° C., and this does not require a large amount of energy to be supplied, since this is the machining temperature. The reaction produces HF, HNO₃, and Na₂TiF₆. The content of the reactor 1 is cooled to a temperature selected in the range of 0° C. to 15° C., preferably to about 5° C., in order to enhance the precipitation of Na₂TiF₆.

Alternatively, the content of the reactor 1 is initially transferred into a settling tank (not shown), which is likewise cooled. The solubility of the Na₂TiF₆ salt can thus be reduced by nearly one half. Under the conditions described, precipitation and settling eliminate about 95% by weight of Ti in the form of Na₂TiF₆(H₂O). Dissolved Na₂TiF₆ may thus remain in the solution, at a quantity of about 5%. The precipitate that is formed and that contains essentially solvated Na₂TiF₆ is eliminated as sludge from the bottom of the reactor 1.

Certain titanium alloy metals present in the spent solution are also eliminated, at least in part, by precipitating and settling, since they form complexes with the regeneration reagents. This applies in particular to aluminum, which precipitates the form of Na₃AlF₆ and is eliminated to more than 99%, of tin (75% elimination), of zirconium (61% elimination), and of iron (18% elimination).

The major fraction of water in the sludges is then removed in a filter 11, and the sludges are then stored prior to being eliminated. Techniques for elimination, including drying, recycling, or other treatments, do not come within the ambit of the present invention. Nevertheless, it can be understood that the volumes of waste for treatment are significantly less than the volumes of waste from conventional chemical etching.

The supernatant is a regenerated solution, that contains essentially HF and HNO₃, together with some titanium and Na⁺ that have not reacted, and finally a proportion of Na₂TiF₆ that remains partially soluble. This solution is transferred into the tank 2 before being returned to the machining vessel 100.

In order to eliminate any crystals that might form by precipitation of the Na₂TiF₆ still present in the solution, it is possible to arrange respective filters 20, 21 at the inlet and/or the outlet of the tank 2. By way of example, it is possible to use a membrane filter, selected to retain particles of size greater than 10 μm, and preferably greater than 5 μm. As shown by the image given in FIG. 2, the crystals of Na₂TiF₆ are of a size enabling these filter means to be effective.

Prior to being poured into the machining vessel 100, the regenerated solution is heated to a temperature identical to that of the etching bath, i.e. about 25° C. to 45° C. The etching bath thus returns more quickly to its operating conditions, such that the machining process need not be interrupted for more than one hour. It can thus operate in quasi-continuous manner.

The volume of regenerated solution that is returned to the machining vessel 100 is determined on the basis of measuring the concentrations of the various species present in the regenerated solution and in the etching bath. After adding said volume of regenerated solution, new measurements are taken and adjustments are made, either by adding water, or by adding HNO₃ and/or HF.

In general manner, concentrations are measured regularly at various points in the equipment, thereby serving to monitor and to control the various steps of the process, automatically, continuously, and in reliable manner.

Simultaneously, it is verified that there is no excessive accumulation of sodium in the machining vessel 100, so as to avoid any risk of undesirable secondary reactions. If sodium exceeds a previously set limit value, e.g. 7 g/L or preferably 5 g/L, a fraction of the spent bath is offloaded into and offloading vessel 120.

Also, certain alloyed metals are not eliminated during regeneration. This applies in particular to vanadium, which becomes concentrated little by little in the etching bath. Its concentration is monitored, and if it reaches an excessive level, e.g. set in the range of 5 g/L to 10 g/L, a fraction of the spent bath is offloaded to the offloading vessel 120.

It is possible to adopt another approach for avoiding undesirable compounds accumulating in the etching bath, which approach consists in regularly eliminating a fraction of the bath at predefined time intervals or at particular times when certain operations are performed. For example, it is possible to eliminate a fraction of the etching bath prior to each occasion when spent solution is drawn off to be regenerated. The fraction of the bath that is to be eliminated may be greater or smaller, e.g. 5% or 10%, depending on the nature of the parts being machined and on the nature of the treatment. The concentrations, in particular of vanadium and of sodium, are thus maintained at levels that are moderate. Such concentrations are thus measured more for monitoring purposes than for triggering emptying.

The presently described method of regenerating the bath is performed in discontinuous manner, by drawing of the spent solution at the desired time and by triggering a regeneration cycle when a certain volume needs to be treated, followed by storing the regenerated volumes of solution in the tank 2. It is also possible to have recourse to equipment that makes use of two reactors 100 operating in alternation. While one spent solution is being regenerated, another is being reintroduced into the production bath. Whatever the system adopted, chemical machining is interrupted only for a short length of time, of the order of one hour. This discontinuous regeneration system is simple to install and requires few resources.

It is also possible to make use of a continuous regeneration device. Such a system makes it possible to avoid interrupting production during regeneration, but at a cost that is higher and while being more complicated to put into place.

Example 2: Efficiencies

Tests of regeneration by the method of the invention have been carried out on a bath containing 25.2 g/L of dissolved titanium. The contents of HNO₃ and of HF in the bath were measured as 1.55 moles per liter (mol/L) and 0.81 mol/L respectively, i.e. levels close to the minimum acceptable values in a preferred implementation of the invention (1.4 mol/L and 0.5 mol/L respectively).

The tests were carried out using three protocols that differ by varying the regeneration reagents: a) NaNO₃ only, b) NaF only, and c) NaNO₃ and NaF, both reagents being added at molar concentrations which are equals. The regeneration reagents were added in stoichiometric quantities minus 5%.

A portion of the spent bath was taken off and placed in a container at 35° C. under stirring for a period of 2 h, in the presence of one or more reagents, added in stoichiometric quantities. Thereafter the reaction mixture was cooled to 5° C. and allowed to settle. The resulting precipitate was separated from the supernatant. The quantity of titanium in the supernatant (i.e. a regenerated solution) was measured and its decrease (reduction) compared with the initial quantity was calculated. The results are given in Table 2.

TABLE 2 Reduction of titanium after regeneration a) NaNO₃ b) NaF c) NaNO₃ + NaF Quantity of 170.4 g/L 84.0 g/L 85.2 g/L of NaNO₃ reagent (s) (i.e. 1 mol/L) and 42. g/L of NaF (i.e. 1 mol/L) Final Ti 4.5 g/L 3.8 g/L 3.3 g/L concentration Reduction 82% 85% 87%

In test c), the contents found in the regenerated solution were 1.9 mol/L of HNO₃ and 1.16 mol/L of HF, which are higher than the maximum acceptable limit values in said preferred implementation (respectively 1.8 mol/L and 1 mol/L), but that is not an impediment since the regenerated solution is subsequently diluted on being returned into the machining vessel. The regenerated solution thus adds a significant quantity of regenerated etching reagents to the bath.

The above-described results are very encouraging, even though they have not been optimized. They could be improved by adjusting production condition parameters.

Example 3: Influence of Sodium

Tests were carried out to determine the influence of sodium on parts subjected to chemical etching. The question put was to verify that the sodium-containing reagents used for regeneration do not degrade the quality of the machined parts. Specifically, as mentioned above, reactions (3) and (4) are not total, such that there always remains a certain quantity, even if minimal, of the reagents NaF and NaNO₃ in solution. These compounds are of small size and cannot be eliminated by filtering. They thus end up in the etching bath.

A test piece of TA6V titanium-vanadium alloy having dimensions of 5 cm×5 cm and a thickness of 3 mm, as standardized for hydrogen embrittlement tests and identical to the test pieces used for monitoring chemical machining treatments in mass production, was used under the same conditions as for conventional monitoring of production conditions: removal of material by machining in order to achieve a thickness of 0.6 mm. The test piece was immersed in a chemical machining bath containing nitric acid and sodium fluoride at concentrations of 1.6N (i.e. 100.8 g/L) of HNO₃ and of 0.8N (i.e. 22.5 g/L) of NaF. Thus, in that bath, hydrofluoric acid HF was replaced with NaF, at a concentration that was so high as to be unreachable in the context of the regeneration method of the invention. The etching bath did not contain any dissolved titanium, so that sodium could not react therewith and remained in free form in the bath. The test was thus carried out under conditions that were extremely unfavorable.

The test piece as machined in that way was subjected to hydrogen analysis, to an intergranular etching test, and to a pitting test. The conclusions of the laboratory were as follows: there was no intergranular etching, no pitting, and no hydrogen embrittlement. For each of the tests, the test piece complied with the requirements of the technical standards defined by the aviation industry for chemical machining of titanium alloys.

Those tests thus show that sodium, as is present in the presently-described technology, does not embrittle parts subjected to chemical etching. The solubility of NaF in the regenerated solution and returning to the chemical machining bath therefore does not degrade the health of parts that are machined chemically. 

1. A method of regenerating a nitric and hydrofluoric acid bath for chemically etching parts made of titanium or titanium alloy and contained in a machining vessel (100), the method comprising determining whether said etching bath is spent, and if so, in performing the steps consisting in: a) transferring a portion of the spent etching bath, referred to as the “spent” solution, from the machining vessel (100) into a reactor (1); b) adding a quantity of NaF and a quantity of NaNO₃ to the spent solution, and allowing it to react to form HF, HNO₃, and Na₂TiF₆; c) settling to separate the resulting precipitate from the supernatant; d) transferring the supernatant, which is a regenerated solution, into a tank (2); e) measuring the concentrations of HF, of HNO₃, and of dissolved titanium in the tank (2) and in the machining vessel (100); and f) determining the volume of regenerated solution that can be added to the spent etching bath in order to obtain a regenerated bath in which the concentrations of HF, of HNO₃, and of dissolved titanium lie in respective predefined acceptable concentration ranges, and transferring said volume of regenerated solution into the machining vessel (100).
 2. A method according to claim 1, wherein in step b), NaF and NaNO₃ are added in quantities that are proportional to the molar quantities that correspond to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium.
 3. A method according to claim 1, wherein in step b), NaF and NaNO₃ are added in quantities that are 1% to 8% less in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium.
 4. A method according to claim 1, wherein in step b), the mixture is allowed to react under stirring for a period of 2 h to 4 h at a temperature lying in the range 25° C. to 40° C.
 5. A method according to claim 1, wherein in step c), the settling is performed at a positive temperature that is less than or equal to 15° C.
 6. A method according to claim 5, wherein the settling is performed in the reactor (1), or after the content of said reactor has been transferred into a settling vessel.
 7. A method according to claim 1, wherein the resulting regenerated solution is filtered at the inlet or at the outlet of said tank, using a device (20, 21) suitable for retaining chemical species of size greater than 5 μm.
 8. A method according to claim 1, wherein the regenerated solution is heated to a temperature identical to the temperature of the machining bath prior to being poured into the machining vessel (100).
 9. A method according to claim 1, wherein: said range of concentrations that are acceptable for HF in the machining bath extends from 0.5N to 1N; and said range of concentrations that are acceptable for HNO₃ in the machining bath extends from 1.4N to 1.8N.
 10. A method according to claim 1, wherein at said range of concentrations that are acceptable for titanium in the machining bath extends from 10 g/L to 40 g/L.
 11. A method according to claim 1, wherein said range of concentrations that are acceptable for titanium in the machining bath extends from 18 g/L to 25 g/L.
 12. A method according to claim 1, wherein it is determined whether the etching bath is spent by the operations consisting in: measuring the concentration of dissolved titanium in the etching bath; comparing said measured concentration with said predefined range of acceptable concentrations; and if said measured concentration is greater than the maximum value of said range, triggering step a) of transferring a portion of the spent etching bath from the machining vessel to said reactor.
 13. A method according to claim 1, wherein the concentration of sodium in the etching bath is measured, and if said sodium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel.
 14. A method according to claim 1, wherein said predetermined limit value for the concentration of sodium in the etching bath is no greater than 7 g/L.
 15. A method according to claim 1, wherein the concentration of vanadium dissolved in the etching bath is measured, and if said vanadium concentration is greater than a predetermined limit value, a fraction of the etching bath is removed from the machining vessel.
 16. A regeneration method according to claim 1, wherein the nitrate and hydrofluoric acid chemical etching bath is a bath for chemically machining or a bath for pickling parts made of titanium or titanium alloy.
 17. A method according to claim 1, wherein in step b), NaF and NaNO₃ are added in quantities that are 5% less, in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium.
 18. A method according to claim 1, wherein said predetermined limit value for the concentration of sodium in the etching bath is less than 5 g/L.
 19. A method according to claim 2, wherein in step b), NaF and NaNO₃ are added in quantities that are 1% to 8% less in molar terms than the molar quantities corresponding to stoichiometric reactions of NaF and of NaNO₃ with the dissolved titanium.
 20. A method according to claim 2, wherein in step b), the mixture is allowed to react under stirring for a period of 2 h to 4 h at a temperature lying in the range 25° C. to 40° C. 